Conventionally, examples of optical pickup apparatus include devices for recording and reproducing information using information recording media such as CDs which use light flux with wavelength of about 780 nm or DVDs which use light flux with wavelength of about 635-650 nm. The optical pickup apparatus records or reproduces information by passing a light flux emitted from a light source through an optical element such as collimator lens or objective lens and converging the light flux on the recording surface of the information recording medium.
In recent times, high density DVDs used with light flux with wavelength of about 405 nm, such as AOD (Advanced Optical Disc) or Blu-ray discs have been developed as the information recording medium used in these optical pickup apparatus.
In the optical pickup apparatus that corresponds to the high density DVD, by forming the antireflection film of the optical element by laminating plural lower refractive index layers and plural higher refractive index layers, the light stability of the optical element is improved. (See Japanese Patent Application Laid-Open No. 2005-11494 publication). In addition, in the optical pickup apparatus in which three types of information recording media which are CD, DVD and high density DVD are compatible, by forming the antireflection film by laminating plural lower refractive index layers and plural higher refractive index layers, reflection can be prevented for the light flux of each of the three wavelengths and the diameter of the converged spot can be reduced, and the balance in the amount of light transmitted can be kept favorable. (See Japanese Patent Application Laid-Open No. 2005-31361 publication).
However, in the antireflection film disclosed in the aforementioned Japanese Patent Application Laid-Open No. 2005-11494 publication and Japanese Patent Application Laid-Open No. 2005-31361 publication, because only layers which have the same composition is used for the lower refractive index layer, of the film stress on the antireflection film due to changes in environmental conditions such as temperature and humidity, repeated film stress on the lower refractive index layer sometimes cause cracking and peeling in the antireflection layer. That is to say the antireflection film has poor environmental stability.
An object of the present invention is to provide an optical element and optical pickup apparatus which has better environmental stability than those of the prior art.